Change checkpoint save default behavior

[jeswan]

This PR closes #1198 by deleting the checkpoint at the end of training by default.

[codecov]

Codecov Report

Merging #1233 (3e753cf) into master (838cdd2) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1233   +/-   ##
=======================================
  Coverage   56.56%   56.56%           
=======================================
  Files         147      147           
  Lines       10582    10582           
=======================================
  Hits         5986     5986           
  Misses       4596     4596           

Impacted Files	Coverage Δ
jiant/proj/main/runscript.py	73.04% <100.00%> (ø)
jiant/proj/simple/runscript.py	80.32% <100.00%> (ø)

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